This application is a 371 of PCT/JP03/09825 Aug. 1, 2003
The present invention relates to a magnetoresistive effect element having an arrangement to obtain a magnetoresistive change by causing an electric current to flow in the direction perpendicular to the layer surface and a magnetic memory device including the magnetoresistive effect element.
Information communication devices, in particular, personal small devices such as personal digital assistants are making great spread, elements such as memories and logics comprising information communication devices are requested to have higher performance such as higher integration degree, higher operation speed and lower power consumption. In particular, technologies for making nonvolatile memories become higher in density and larger in storage capacity are progressively increasing their importance as technologies for replacements of hard disk and optical disc that cannot be essentially miniaturized because they have movable portions.
As nonvolatile memories, there may be enumerated flash memories using semiconductors and an FRAM (Ferro electric Random Access Memory) using ferroelectric material and the like.
However, the flash memory encounters with a drawback that its write speed is as slow as the microsecond order. On the other hand, it has been pointed out that the FRAM has a problem in which it cannot be rewritten so many times.
A magnetic memory device called an MRAM (Magnetic Random Access Memory) described in “Wang et al., IEEE Trans. Magn. 33 (1977), 4498”, receives a remarkable attention as a nonvolatile memory which can overcome these drawbacks. Since this MRAM is simple in structure, it can easily be integrated at a higher integration degree. Moreover, since it is able to record information based upon the rotation of magnetic moment, it can be rewritten so many times. It is also expected that the access time of this magnetic random access memory will be very high, and it was already confirmed that it can be operated at the access time of nanosecond order.
A magnetoresistive effect element for use with this MRAM, in particular, a tunnel magnetoresisitve effect (Tunnel Magnetoresistance: TMR) element is fundamentally composed of a lamination layer structure of a ferromagnetic tunnel junction of ferromagnetic layer/tunnel barrier layer/ferromagnetic layer. This element generates magnetoresistive effect in response to a relative angle between the magnetization directions of the two magnetic layers when an external magnetic field is applied to the ferromagnetic layers under the condition in which a constant current is flowing through the ferromagnetic layers. When the magnetization directions of the two magnetic layers are anti-parallel to each other, a resistance value is maximized. When they are parallel to each other, a resistance value is minimized. Functions of memory element can be demonstrated by creating the anti-parallel state and the parallel state with application of the external magnetic field when the magnetization direction of one ferromagnetic layer is inverted.
In particular, in a spin-valve type TMR element, when one ferromagnetic layer is antiferromagnetically coupled to the adjacent antiferromagnetic layer, it is set to the magnetization fixed layer of which magnetization direction is constantly made constant. The other ferromagnetic layer is set to the magnetization free layer of which magnetization direction is easily inverted with application of an external magnetic field and the like. Then, this magnetization free layer becomes an information recording layer in a magnetic memory.
In the spin-valve type TMR element, its resistance changing ratio is expressed by the following equation (A) where P1, P2 represent spin polarizabilities of the respective ferromagnetic layers.2P1P2/(1-P1P2)  (A)
As described above, the resistance changing ratio increases as the respective spin polarizabilities increase. With respect to a relationship between materials for use with ferromagnetic layers and this resistance changing ratio, ferromagnetic chemical elements of Fe group such as Fe, Co, Ni and alloys of three kinds thereof have already been reported so far.
Information is read out from the TMR element of the MRAM based upon a difference current with application of a constant bias voltage or a difference voltage with application of a constant bias current in the state of “1”, for example, obtained when directions of magnetic moments of one ferromagnetic layer and the other ferromagnetic layer sandwiching the tunnel barrier layer are anti-parallel to each other and a resistance value is high and in the state of “0” obtained when the directions of the magnetic moments are parallel to each other.
Accordingly, a higher TMR ratio (magnetoresistive changing ratio) is advantageous, and hence a high-speed memory having a high integration degree and a low error rate can be realized.
In addition, the TMR element having the ferromagnetic layer/tunnel barrier layer/ferromagnetic layer has a bias voltage dependence of TMR ratio, and it is known that the TMR ratio decreases as the bias voltage increases. Since it is known that, in most cases, a read signal takes a maximum value at a voltage (Vh) in which a TMR ratio decreases to the half depending on the bias voltage dependence when information is read out the magnetic memory by a difference current or a difference voltage, a small bias voltage dependence is effective for decreasing read errors.
The MRAM includes switching elements such as transistors to select a TMR element, in addition to the above-mentioned TMR element, and has a semiconductor circuit including the switching element.
When such semiconductor circuit and the TMR element coexist within the same chip, since a semiconductor circuit manufacturing process requires a process for heating the chip at temperature in excess of 350° C., the TMR element needs similar temperature durability.
However, it is known that a TMR element having a ferromagnetic layer made of alloy of Fe-group chemical element such as Fe, Co and Ni is considerably deteriorated in magnetoresistive changing ratio at temperature higher than about 300° C., and therefore it has a problem from a heat-resisting property standpoint. This magnetoresistive changing ratio may be deteriorated by undesired impurities entered into the ferromagnetic layer or the tunnel barrier layer after components of layers comprising the TMR element have been mutually diffused by heat.
Therefore, when the magnetization free layer is made of amorphous alloy in which B, Si, C, P, Al, Ge, Ti, Nb, Ta, Zr, Mo are added to the alloy of the Fe-group chemical element such as Fe, Co and Ni, the magnetoresistive changing ratio can be improved and the magnetization direction can be inverted with stability so that read characteristics in the MRAM can be improved.
However, when such amorphous alloy is heated at temperature higher than its crystallization temperature, magnetic characteristics, those requested for the TMR element for use with MRAM, such as magnetoresistive changing ratio are deteriorated.
As described above, in order to realize the MRAM that can make the excellent read characteristics and high affinity of the semiconductor circuit manufacturing process become compatible with each other, the magnetic characteristics (high magnetoresistive changing ratio, etc.) of the TMR element should be guaranteed after the magnetic element has experienced relatively high temperature. For this reason, it has been requested so far to improve heat-resisting property of the TMR element.
In order to solve the above-mentioned problems, it is an object of the present invention to provide a magnetoresistive effect element having satisfactory magnetic characteristics in which deterioration of a magnetoresistive changing ratio due to annealing can be suppressed and a magnetic memory device including this magnetoresistive effect element and which has excellent write characteristics.